This invention relates to methods for forming protective films and, more particularly, to a new and improved method for forming a protective film on the surface of a semiconductor body in order to prevent a semiconductor device from unwanted deterioration.
One example of a protective film on a semiconductor is an oxide which is formed at high temperature on a surface of a semiconductor substrate. Where a silicon crystal, for example, is used as a semiconductor substrate, an oxide film is produced by placing the silicon crystal in a water vapor while heating it to above 1000.degree. C. This causes a chemical reaction at the silicon crystal surface to produce an oxide film thereon. Another example is a nitride film formed at high temperature. Such a nitride film is formed by placing a semiconductur substrate, for example, a silicon crystal, in a gas containing nitrogen, such as ammonia, while heating it at a high temperature of above 800.degree. C. This causes a chemical reaction at the silicon crystal surface to provide a nitride film thereon.
One disadvantage of a silicon oxide (SiO.sub.2) protective film is that it can react with hydrofluoric acid and then decompose. Therefore, an SiO.sub.2 film cannot be employed when it is likely to come into contact with a solution or vapor containing hydrofluoric acid. On the other hand, a silicon nitride (Si.sub.3 N.sub.4) film will decompose when exposed to heated concentrated phosphoric acid. Accordingly, an Si.sub.3 N.sub.4 film cannot be used when it is likely to come into contact with a solution containing phosphoric acid or its vapor.
An object of the present invention is to provide a method for forming a chemically resistive protective film on a semiconductor body in order to avoid difficulty in subsequent surface treatments resulting from reaction of the protective film with chemicals used in manufacturing processes for semiconductor devices.